The Basal Friction Coefficient of Granular Flows With and Without Excess Pore Pressure: Implications for Pyroclastic Density Currents, Water-Rich Debris Flows, and Rock and Submarine Avalanches

Numerous large-scale geophysical flows propagate with low-apparent basal friction coefficients, but the source of such phenomenology is poorly known. Motivated by scarce basal friction data from natural flows, we use numerical methods to investigate the interaction of granular flows with their substrate under idealized conditions. Here we investigate 3-D monodisperse and polydisperse fluid-particle granular flow rheology and flow-substrate interaction using discrete element modeling and coarse-graining techniques. This combination allows us to calculate the continuum fields of solid fraction, velocity, shear stress, and solid pressure and compare it with force measurements on the substrate. We show that the wall/basal friction coefficient is not constant. Instead, it is a function of the nondimensional slip defined as the ratio of the slip velocity over the slip velocity fluctuations. The scaling of the wall friction with nondimensional slip is independent of air viscosity and density and presence of excess pore pressure. Therefore, the reduction of the basal stress that must occur in mobile natural flows with excess pore pressure is not ascribed to the lowering of wall friction coefficient. Instead, lowering of the normal stress by fluid drag in flows with elevated pore fluid pressure justifies the definition of effective wall and internal friction coefficients to capture the geophysical flow rheology and the forcing on its substrate. These results are fundamental to understand the dynamics of geophysical mass flows including pyroclastic density currents, water-rich debris flows, and rock and submarine avalanches.